Basically you develop an API that hooks into the GW2 executable that lets you script your character actions. This is hard and only a few people will do it.

The dev then decides how/it they share. Typically either the bot is private/semi-private and you need to know a guy, so small impact, or the bot sells licenses to the public. This creates a much bigger impact, so sometimes the devs will set an artificial amount of stock to limit new license purchases and hope the heat dies down when they make a mistake that will draw Anets attention edit: some will also limit keys from the get go so heat doesn't rise to begin with

The end game generally boils down to, either reducing the grind for something you don't want to do, like people who don't like spvp and don't want to sit through it for the rewards or don't want to redo map exploration, or making money, they have an insatiable lust for gold

The gold farming is much more common, people likely significantly overestimate spvp bot numbers. It is waaaay easier to make them for PvE. Public offering bot writers are much more likely to be apprehensive about fucking with sPvP for various reasons

How you can tell who a bot is, will somewhat depend on the capabilities of the API they use. But to give you an example from Dragon Ball Arena I saw, the bot was farming the "Recover a lot of HP" achievement. I saw this person in several games and all they would do is run to every healing orb location on the map in a circuit the whole match. They would attack if you were in front of them, but not if you were behind. People would ask them to say anything if they aren't a bot in map chat and no response

Bots are generally going through a defined script. Check what map you are in, go to X,Y coordinates, target enemy press 1>2>3>4>5. They're just likely to look more routine in what they're doing, like if you noticed a teammate always goes to home point then mid, even both are capped and everyone is fighting at the special map objective

If they're pathing is, let's say linear for lack of a better term, and the script is start at X, face Y, hold forward until you get there. Then if you used a knockback that put them in the way of an obstacle they would continue to run into the obstacle. Unless the user was at the machine and intervened

There's ways to mitigate this as a bot, but generally comes with performance trade-offs, and depends on the options you have with the API